docs: def to_gsea(graph: BELGraph, file: Optional[TextIO] = None) -> None:
      """Write the genes/gene products to a GRP file for use with GSEA gene set enrichment analysis.
  
      .. seealso::
  
          - GRP `format specification <http://software.broadinstitute.org/cancer/software/gsea/wiki/index.php/Data_formats#GRP:_Gene_set_file_format_.28.2A.grp.29>`_
          - GSEA `publication <https://doi.org/10.1073/pnas.0506580102>`_
      """
      print('# {}'.format(graph.name), file=file)
      nodes = {
          data[NAME]
          for data in graph
          if NAMESPACE in data and data[NAMESPACE].upper() == 'HGNC' and NAME in data
      }
      for node in sorted(nodes):
          print(node, file=file)